19-year-old Hun Joon "Paul" Lee boarded the bus that was supposed to take him to a transition program at the Sierra Education Center on the morning of September 12, but he never made it to school and he never came home. His body was found hours later still inside the bus, which was parked at the bus depot.

The bus driver, 36-year-old Armando Able Ramirez, was taken into custody on Wednesday and charged with a felony count of dependent adult abuse in connection with Lee's death.

According to prosecutors, Ramirez believed that Lee had gotten off the bus to go to school that morning, but did not check to see if anyone was left in the vehicle at the end of his shift. All of the windows on the bus were close and the temperature that day was over 90 degrees.

Investigators determined that Lee had not been able to verbally communicate and needed special care.

Ramirez — a substitute driver for Lee’s bus who was working a split shift — apparently believed Lee had gotten off the bus to go to school that morning, but did not walk to the back of the bus or look over his shoulder to check if anyone was left in the vehicle at the end of his morning shift, according to prosecutors.

He returned the bus to the bus yard, filled out paperwork, left for home and returned for work that afternoon, when he was notified by a dispatcher that Lee was missing, prosecutors said.

Ramirez went back to the bus, found Lee unresponsive and called for help, auhorities said. Paramedics performed CPR, but Lee was pronounced dead at the scene.

If convicted, Ramirez could face up to nine years in prison.

Lee's parents have also filed a wrongful death lawsuit against the company that operated the bus, Pupil Transportation Cooperative, alleging that employees failed "to do a mandatory and routine sweep of the subject bus prior to exiting, especially considering that there were only three children on the subject bus who were of special needs."